For instance, water boils at 212°F at atmospheric
pressure (14.7 psia), as shown in view A, figure 6-7.
The same water boils at 228°F if the pressure is raised
5.3 psig (20 psia), as shown in view B, figure 6-7. On
the other hand, the same water boils at 32°F in a partial
vacuum of 29.74 inches of mercury (Hg), as shown in
Figure 6-7.A. Water boils at atmospheric pressure; B.
Water boils at 20-psia absolute pressure.
Figure 6-8.Water boils quicker in a vacuum.
This effect of reduced pressure on the boiling
temperature of refrigerants makes the operation of a
refrigeration system possible. The pressure
temperature relationship chart in figure 6-9 gives the
pressures for several different refrigerants.
An increase in the temperature of a refrigerant
results in an increase in pressure, and a decrease in
temperature causes a decrease in pressure. By the
same token, a decrease in pressure results in a
corresponding decrease in temperature.
This means that as the pressure of a refrigerant is
increased, so is the temperature at which the
refrigerant boils. Thus, by regulating the pressure of
the refrigerant, the temperature at which evaporation
takes place and at which the latent heat of evaporation
is used can be controlled.
VAPORIZATION is the process of changing a
liquid to vapor, either by evaporation or boiling. When
a glass is filled with water, as shown in figure 6-10, and
exposed to the rays of the sun for a day or two, you
should note that the water level drops gradually. The
loss of water is due to evaporation. Evaporation, in this
case, takes place only at the surface of the liquid. It is
gradual, but the evaporation of the water can be
speeded up if additional heat is applied to it. In this
case, the boiling of the water takes place throughout
the interior of the liquid. Thus the absorption of heat
by a liquid causes it to boil and evaporate.
Vaporization can also be increased by reducing the
pressure on the liquid, as shown in figure 6-11.
Pressure reduction lowers the temperature at which
liquid boils and hastens its evaporation. When a liquid
evaporates, it absorbs heat from warmer surrounding
objects and cools them. Refrigeration by evaporation
is based on this method. The liquid is allowed to
expand under reduced pressure, vaporizing and
extracting heat from the container (freezing
compartment), as it changes from a liquid to a gas.
After the gas is expanded (and heated), it is
compressed, cooled, and condensed into a liquid again.
CONDENSATION is the process of changing a
vapor into a liquid.
For example, in figure 6-12, a
warm atmosphere gives up heat to a cold glass of water,
causing moisture to condense out of the air and form on
the outside surface of the glass. Thus the removal of
heat from a vapor causes the vapor to condense.